Application of high k dielectric to non volatile memory devices

A STUDY OF NOVEL HIGH- MATERIALS IN FLASH MEMORY DEVICES AND INTEGRATED CIRCUIT METAL-INSULATOR-METAL CAPACITORS ZHANG LU National University of Singapore 2010 A STUDY OF NOVEL HIGH- MATERIALS IN FLASH MEMORY DEVICES AND INTEGRATED CIRCUIT METAL-INSULATOR-METAL CAPACITORS ZHANG LU (B.Eng., National University of Singapore) A THESIS SUBMITTED FOR THE DEGREE OF DOCTOR OF PHILOSOPHY DEPARTMENT OF ELECTRICAL AND COMPUTER ENGINERRING NATIONAL UNIVERSITY OF SINGAPORE 2010 Acknowledgements ACKNOWLEDGMENTS The four years of graduate study in National University of Singapore has been one of the most important periods and led me to firm direction in my life. First and foremost, I would like to express my sincere gratitude to my supervisor advisors, Prof. Chan Siu Hung, Daniel and Prof. Cho Byung-Jin, for their invaluable guidance, encouragement, and kindness , during my postgraduate study at NUS, not only in terms of technical knowledge, but also personally. I will definitely benefit from the experience and knowledge I have gained from them throughout my life. I am especially grateful of Prof. Chan’s help, who provides me with the opportunity to join his research group in the first place. Secondly for his patience and painstaking efforts devoting in my research as well as his kindness and understanding which accompanied me over the last four years. I also greatly appreciate Prof Cho from the bottom of my heart for his knowledge, expertise and foresight in the field of semiconductor technology. Without his guidance, it would be impossible for me to have completed this. I would like to extend my gratitude to other teaching staffs in Silicon Nano Device Lab (SNDL): Associate Professor Ganesh Samudar, Dr. Lee Sunjoo, Dr. Yeo Yee Chia, and Dr. Zhu Chunxiang for sharing of their knowledge. I also owe the opportunity to collaborate with so many talented graduate students in SNDL at NUS. It is my pleasure to have worked with Mr He Wei, Ms Pu Jing, Mr. Hwang Wan Sik, Ms Tan Yan Ny and Mr Kim Sung-Jung, many thanks for their useful suggestions and kind assistance in my research. i Acknowledgements The technical staffs in SNDL are also gratefully acknowledged: Mr. Yong Yu Fu, Patrick Tang, Mr. O Yan Wai Linn, Lau Boon Teck, Dr. Han Geng Quan, Mr Sun Zhi Qiang, it was a joyful experience working with all of them. Many of my thanks also go to other students from SNDL, Andy Lim Eu-Jin, Eric Teo, Fu Jia,Gao Fei , Huang Jidong, Jiang Yu, Li Rui, Lin Jianqiang Ma Fa Jun, Oh Hoon jung, Ren Chi, Rinus Lee, Shen Chen, Tan Kian Ming, Wang Xinpeng, Wang Yinqian, Wu Nan, Yang Jianjun, Yang Weifeng, Yu Xiongfei, Zhang Chunfu, , Zhang Qingchun, and Zhu Ming for their useful discussions and kind assistances during the course of my research, as well as the friendships that will be cherished always. Last but not least, my deepest love and gratitude will go to my family, for their love, patience and support throughout my postgraduate studies. Zhang Lu January 2010 ii Abstract ABSTRACT Rapid scaling of complementary-metal-oxide (CMOS) devices has led to performance challenges posed by the properties of conventional materials. To meet this challenge, new high-κ materials have been widely studied for the applications ranging from memory devices to radio frequency (RF)/ mixed-signal (MS) technology for wireless communication. In this work, we explored the scaling limit as well as possible materials for the interpoly dielectric (IPD) layer in future floating gate flash memory devices. A systematic study of leakage current through the IPD layer was conducted using the MEDICI simulator. Various high-κ dielectrics were studied for their feasibility of being used as IPD. Simulation result shows that while conventional high- materials like Al2O3 and HfO2 can no longer meet the ITRS scaling requirement, new high-κ dielectrics like La doped Al2O3 and HfO2 show the potential to be used in the 32 nm technology node. Experimental result shows that multi-layer high-κ dielectric structures using Al2O3 and HfO2 based dielectric stack exhibit much improved dielectric thermal stability than that of single layer dielectric. Moreover, simulation suggests that, contrary to the conventional high-low-high barrier structure like oxide-nitride-oxide (ONO) IPD, a low-high-low barrier structure like HfO2–Al2O3–HfO2 exhibits lower leakage current at high electrical fields due to the longer effective tunneling distance. The advantage of low-high-low barrier structure over high-low-high barrier structure was then confirmed by experiments. The presence of an interfacial layer between high- and polysilicon floating iii Abstract gate (FG) was studied with simulation and experiment results reveal that a significant portion of the voltage will drop across the low- interfacial layer, decreasing the effective tunneling distance, and leading to an increased leakage current several orders higher. It is suggested that the control of this interfacial layer plays an important role in reducing leakage current. By using a low-high-low barrier structure IPD together and suppressing the interfacial layer between high-k and polysilicon floating gate, the leakage current through IPD layer can be significantly reduced. Novel high-κ MIM capacitors were also developed for radio frequency and analog/mixed-signal (RF and AMS) IC application. The feasibility of a La-doped HfO2 based MIM capacitor was investigated using the dielectric deposited by an atomic layer deposition (ALD) method. It is found that for a single layer HfLaO thicker than 20 nm, the crystallization temperature can be as low as 420oC. A high dielectric constant of 38 is achieved upon film crystallization, however with a trade off degraded voltage linearity. By insertion of a LaAlO3 layer, grain boundary channels extending from the top to the bottom electrode are interrupted, and good interfacial quality near the bottom electrode can be achieved. Consequently, HfLaO film crystallization is effectively suppressed and an improved voltage linearity results. Both single layer 8% La doped HfO2 capacitors and HfLaO-LaAlO3-HfLaO multilayer stacked MIM capacitors exhibit excellent electrical characteristics such as low leakage current, quadratic linearity, high breakdown field and good device reliability, which make them promising candidates for RF circuit applications. iv Table of Contents TABLE OF CONTENTS Acknowledgements . i Abstract .iii Table of Contents v List of Tables . x List of Figures . xi List of Abbreviation xxi Chapter 1. Introduction to Floating Gate Flash Memory Devices 1.1 Semiconductor Memory Devices . 1.2 Flash Memory Devices 1.3 Operating Principles of Floating Gate Devices . 1.3.1 Basic Programming Mechanisms 1.3.2 Basic Erasing Mechanisms . 1.3.3 Basic Reading Operation . 1.4 Challenges in Flash Memory Scaling 1.4.1 Scaling Limits for Conventional Interpoly Dielectric Layers 14 1.4.2 Scaling Limits for Conventional Interpoly Dielectric . 17 1.4.3 Opportunities Arising from Interpoly Dielectric Scaling 19 1.4.4 Previous Work on High-k Dielectrics for Interpoly Dielectric 19 v Table of Contents 1.5 Objectives of the Work 21 1.6 Thesis Outline 21 Reference . 23 Chapter 2. MIM Capacitors for Radio Frequency and Analog/MixedSignal Technologies 26 2.1 RF and Mixed-Signal Technologies . 26 2.2 Metal-Insulator-Metal Capacitor for RF/AMS Application 26 2.3 Parameters of RF/AMS Capacitors 28 2.3.1 Dielectric Constant and Capacitance Density 28 2.3.2 Temperature Coefficient of Capacitance (TCC) 29 2.3.3 Voltage Linearity . 30 2.3.4 Leakage Current . 31 2.3.5 Dissipation Factor and Loss Tangent . 32 2.3.6 Compatibility with BEOL Integration . 33 2.3.7 Technology Trends and Challenges . 33 References 35 Chapter 3. Modeling of Leakage Current in Interpoly Dielectric Layers in Floating Gate Flash Memories . 39 3.1 Introduction . 39 3.2 Simulation Methodology 40 vi Table of Contents 3.3 Results and Discussion . 48 3.3.1 Single Layer IPD with Different Barrier Height 48 3.3.2 Multi Layer IPD Structure . 51 3.3.3 Effect of an Interfacial Layer between the Polysilicon Gate and Dielectric . 59 3.4 Summary . 63 Reference 65 Chapter 4. High-κ Dielectrics for Interpoly-Dielectric Layers . 67 4.1 Introduction . 67 4.2 Device Fabrication 68 4.3 Evaluation of Single Layer High-κ Dielectric 69 4.3.1 Evaluation of Hafnium oxide (HfO2) 70 4.3.2 Evaluation of Tb-doped Hafnium oxide . 71 4.3.3 Evaluation of Lanthanum-doped Hafnium Oxide Films . 76 4.3.4 Evaluation of Yttrium Oxide (Y2O3) Dielectric Films 83 4.4 Multi-Layer High-κ Dielectric Evaluation . 85 4. Suppression of Interfacial Layer 91 4.4 4.5.1 Surface Nitridation of Si . 92 4.5.2 Poly-SiGe Floating Gate . 93 Summary 96 Reference 96 vii Table of Contents Chapter 5. Atomic Layer Deposited HfLaO and LaAlO3 Multilayered Dielectrics for High Performance MIM Capacitors . 101 5.1 Introduction . 101 5.2 Device Fabrication 103 5.3 Single Layer La-Doped HfO2 MIM Capacitors 105 5.3.1 Structural Analysis 105 5.3.2 Leakage Currents Characteristics 112 5.3.3 Capacitance Density and Voltage Linearity 115 5.4 HfLaO-LaAlO3-HfLaO Multi-layer MIM Capacitor 120 5.4.1 Motivation . 120 5.4.2 Experiment 121 5.4.3 Results and Discussion 122 5.5 Summary . 130 Reference . 131 Chapter 6. Conclusions . 134 6.1 6.2 Summary 134 6.1.1 Understanding the Interpoly Dielectric Layer . 134 6.1.2 Lanthanum-doped Hafnium Oxide for RF/MS MIM Capacitors 136 Suggestions for Future Research . 138 Reference . 139 viii Fig.5.20 -3 10 HLH 7.4 fF/m HfLaO 7.5 fF/m -4 10 -4 10 -5 10 -6 10 -7 10 -5 10 -8 10 Bias (V) -6 10 -9 10 Leakage current(A/cm 2) Current Density @ -3.3V (A/cm ) Chapter Atomic Layer Deposited HfLaO and LaAlO3 Multilayered Dielectrics for High Performance MIM Capacitors HfLaO (this work) HLH (this work) HfO2 [14] Tb doped HfO2 [3] HfO2/Al2O3 laminate[15] HfO2/HfxCyNz/HfO2 [11] TiSiO4[2] -2 -4 -6 -8 -10 -12 -7 10 -8 10 -9 10 12 16 Capacitance Density (fF/m ) Leakage current density (at -3.3 V) against capacitance density of HfLaO and HfLaO/LaAlO3/HfLaO MIM capacitors. Bench-marked results were plotted Current Density @ -5.6V (A/cm ) together for comparison purpose. Fig.5.21 -2 10 -3 multi layer single layer -4 420 C FGA 10 o 10 -5 10 -6 10 -7 10 -8 10 -9 10 10 12 14 16 18 Capacitance Density (fF/m ) Leakage current density (at -5.6 V) against capacitance density of HfLaO and HfLaO/LaAlO3/HfLaO MIM capacitors. 125 Chapter Atomic Layer Deposited HfLaO and LaAlO3 Multilayered Dielectrics for High Performance MIM Capacitors Figure 5.22 summaries the quadratic voltage coefficients of capacitance against capacitance density measured at 100 KHz for both HfLaO single layer and HfLaO/LaAlO3/HfLaO multi-layer stacks. The results are also compared to the bench-marked results from MIM capacitors with other materials. It can be observed that the quadratic VCC increases with the capacitance density for both single and multi-layer films. The HLH multi-layer has the quadratic voltage coefficient comparable to the HfO2/Al2O3 laminate structure [15]. The HfLaO single layer capacitor shows degradation in the quadratic voltage coefficient and this is believed to be due to the crystallization of HfLaO. However, it still exhibits quadratic linearity which is better than that of pure HfO2. The HLH multi-layer MIM capacitors can maintain a low quadratic linearity of less than 1000 ppm/V2 up to a capacitance of fF/µm2 after 420oC annealing. As a direct comparison, Fig. 5.23 shows the CV curve for HfLaO and HLH with capacitance density of 7.5 fF/μm2 and 7.45 fF/μm2 respectively. 126 Chapter Atomic Layer Deposited HfLaO and LaAlO3 Multilayered Dielectrics for High Performance MIM Capacitors Quadratic voltage coefficient, ppm/V ) 10 10 HfLaO (this work) HLH (this work) HfO2 [14] Tb doped HfO2[3] HfO2/Al2O3[1,15] 10 Al2O3 [1,16] TiSiO4 [2] Fig.5.22 10 12 14 16 18 Capacitance Density (fF/m ) Quadratic voltage coefficient () of MIM capacitors versus capacitance density of HfLaO and HfLaO/LaAlO3/HfLaO capacitors. Bench-marked results Capacitance Density(fF/m ) from the literature were also plotted . 7.7 7.6 7.5 7.4 7.3 Fig.5.23 HLH 7.4fF/m HfLaO 7.5fF/m -2 -1 Bias (V) Dependence of the Capacitance on Voltage Bias for HfLaO and HLH MIM capacitors with similar capacitance density. 127 Chapter Atomic Layer Deposited HfLaO and LaAlO3 Multilayered Dielectrics for High Performance MIM Capacitors To further study the reliability of the MIM capacitors, constant voltage stress tests were performed for both HfLaO single layer and HLH multi-layer MIM capacitors. TaN metal gate is used for top and bottom electrode, various negative biases were applied on the top electrode to study the reliability characteristics. The median time-to-failure (T50%) is shown in Fig. 5.24. For HLH multi-layer MIM capacitor with a capacitance of 7.4 fF/µm2, the projection of the operation voltage for a 10-year-lifetime is -5.2 V. On the other hand, HfLaO single layer MIM capacitor with a capacitance density of 7.5 fF/µm2 shows the projected operation voltage for 10-year-lifetime is -7.7 V. Such advantage of HfLaO single layer is attributed to its thicker physical thickness for the same capacitance because of the very high  value. The relationship between the extrapolated operating voltage for 10-year lifetime and capacitance density is shown in Fig. 5.25, which clearly indicates the advantage of crystallized HfLaO single layer in terms of long term reliability. 128 Chapter Atomic Layer Deposited HfLaO and LaAlO3 Multilayered Dielectrics for High Performance MIM Capacitors Time-to-Breakdown (s) 10 10 years 10 HLH 7.4fF/cm 10 HfLaO 7.5fF/cm 10 10 10 10 10 10 10 -4 -6 -8 -10 -12 -14 -16 Stress Voltage (V) Life time projection of HfLaO single layer (7.5 fF/µm2) and Fig.5.24 HfLaO/LaAlO3/HfLaO multi-layer (7.4 fF/µm2) MIM capacitors measured at room Operating Voltage Projection for 10-year Lifetime (V) temperature. Fig.5.25 HLH HfLaO 10 12 14 Capacitance Density (fF/m ) 16 Comparison of reliability performance of HfLaO single layer and HfLaO/LaAlO3/HfLaO multi-layer MIM capacitors. 129 Chapter Atomic Layer Deposited HfLaO and LaAlO3 Multilayered Dielectrics for High Performance MIM Capacitors 5.5 Summary The dielectric properties of single layer 8% La-incorporated HfO2 as well as HfLaO/LaAlO3/HfLaO multi-layer stack have been investigated to assess their potential for MIM capacitor applications. The HfLaO single layer MIM capacitors demonstrate a low leakage current density of less than 1×10-7 A/cm2 at -3.3 V with a capacitance density of up to 16 fF/µm2. On the other hand, HfLaO/LaAlO3/HfLaO multi-layer stack shows a low voltage linearity (α[...]... well, but offsetting limitations can prevent the product from becoming a genuine solution, especially in newer applications Thanks to the characteristics of non- volatile memory, it offers a wide range of applications from industrial computers to consumer handheld devices 2 Chapter 1 Introduction to Floating Gate Flash Memory Devices 1.2 Flash Memory Devices Among various non- volatile memory devices, ... read-only memory (EEPROM), and flash memories These devices will keep stored information even when the power supply is switched off Table 1.1 lists some of the volatile and non- volatile memory types and their related features 1 Chapter 1 Introduction to Floating Gate Flash Memory Devices Table 1.1 Volatile and non- volatile memory types and their main features Memory Type Features Volatile Memory Dynamic...Table of Contents Appendix List of Publications 140 ix List of Tables LIST OF TABLES Table 1.1 Volatile and non- volatile memory types and their main 2 features Table 3.1 Electrical characteristics of different dielectric materials 48 Table 5.1 Comparison of various high capacitance density MIM 120 capacitors using high- κ dielectrics x List of Figures List of Figures Fig 1.1... (FG) type flash memory devices are the core of every modern non- volatile semiconductor memory device It has become the preferred device because its ease of erasing stored charge enables a memory device that is both non- volatile and reprogrammable Early floating gate devices had to be erased with a few minutes of ultraviolet radiation which imparts enough energy for stored electrons to surmount the... floating gate memory, whole blocks of devices are erased electrically in less than a second, giving rise to the term flash memory [2, 3] This change greatly simplified memory packaging and simultaneously opened up vast new sets of applications [2, 4] Today, more than 90% of non- volatile memory production is based on the floating gate concept [5] Meanwhile, specific applications have stimulated to the development... characteristic of a floating gate device when there are no electrons stored (“1”) and when electrons are stored (“0”) in the floating gate 1.4 Challenges in Flash Memory Scaling The increasingly fast growth in the non- volatile memory market is driven mainly by mobile applications which provide the market push for the rapid 9 Chapter 1 Introduction to Floating Gate Flash Memory Devices development of non- volatile. .. development of a wide variety of flash memory devices, and they all show different characteristics according to the structure of the selected cell and the complexity of the array organization There are two major kinds of flash memories, NOR and NAND, as depicted in Fig.1.1 [3] In NOR memory, each cell in a memory array is directly connected to the word-lines and bit-lines of the memory array, while NAND memory. .. Random Access Memory (DRAM) High density, low cost, high- speed, high power Static Random Access Memory (SRAM) Highest speed, high- power, low-density memory; limited density drives up cost Non- Volatile Memory Electrically Erasable Programmable Read-only Memory (EEPROM) Electrically byte-erasable; lower reliability, higher cost, lowest density Electrically Programmable Read-only High- density memory; must... Al2O3 – HfO2 low -high- low barrier structure shows a lower leakage current than xvi List of Figures Al2O3 – HfO2– Al2O3 high- low -high structure Fig 4.16 Leakage current comparison of HfO2 – Al2O3 – HfO2 multilayer 89 stacks with different Al2O3 middle layer EOT percentage It is shown that samples with Al2O3 thickness closes to 80% shows lower leakage current than samples with Al2O3 thickness of 50% Inset... diffraction xxiii Chapter 1 Introduction to Floating Gate Flash Memory Devices Chapter 1 Introduction to Floating Gate Flash Memory Devices 1.1 Semiconductor Memory Devices Semiconductor memories devices have been around for many decades Their areal density has increased almost four times every three years and they are now used in many new applications, where both high data retrieval speed and lower power . Chapter 1. Introduction to Floating Gate Flash Memory Devices 1 1.1 Semiconductor Memory Devices 1 1.2 Flash Memory Devices 3 1.3 Operating Principles of Floating Gate Devices 4 1.3.1 Basic. requirements of MIM capacitors to 2022 according to ITRS 2007. 29 Fig. 3.1 Device structure of a floating gate type flash memory device. 41 Fig. 3.2 Energy band diagram of three main types of carrier. Comparison of various high capacitance density MIM capacitors using high-κ dielectrics. 120 List of Figures xi List of Figures Fig. 1.1 Circuit schematics and top-down memory cell